DC-DC Power Conversion
For a DC-DC power conversion, switching mode buck voltage converter is typically used to convert an input voltage from a power supply to a converter output voltage that is connected to a load such as a central processing unit (CPU). For the converter to have better performance as well as to reduce the voltage difference between the input voltage and the converter output voltage, multi-phase or multi-channel scheme is employed thereof, by which the input current from the power supply is spilt into a plurality of channel currents. For examples of such system, readers are referred to U.S. Pat. No. 6,278,263 issued to Walters et al. and U.S. Pat. No. 6,414,470 issued to Liu et al.
Basic Scheme
Further to the above introduction to DC-DC power conversion, a circuit 10 is provided in FIG. 1 to show a simplified single-channel or single-phase voltage converter or one channel of a multi-phase or multi-channel voltage converter to illustrate the basic principles of the power conversion thereof, which is also known as the switching mode buck voltage converter, and by which a converter output voltage Vout is generated from a power source or supply voltage Vin under the conversion that alternatively switches high-side and low-side switches 12 and 14, such as MOS transistors, to regulate its output Vout in conjunction with an output inductor 16 and an output capacitor 18. The high-side and low-side switches 12 and 14 are alternatively turned on and off by a clock 20 and its complementary signal 22 whose ON-duty is determined in accordance with the ratio of the output voltage to the input voltage of the converter, i.e., Vout/Vin. During the ON-duty of the clock 20, the high-side switch 12 is turned on and the low-side switch 14 is turned off. In contrast, during the OFF-duty of the clock 20, the high-side switch 12 is turned off and the low-side switch 14 is turned on. By this manner the desired converter output voltage Vout is obtained and a current IL is supplied to the load through the inductor 16.
It is also shown in FIG. 1 a diode 24 with dashed line to replace the low-side switch 14 in some other applications, e.g., asynchronous voltage regulator. However, the principles of the power conversion are the same for such modifications.
Performance
In the conversion system 10 shown in FIG. 1, since the inductor 16 is inserted between the converter output Vout and the switches 12 and 14, it will influence the performance of the converter output Vout, especially on the transient state operations. Ripple current and response speed are two most important factors to describe the system performance thereof. For the inductor 16, as shown in FIG. 2 by curves 26 and 28 for the steady state, a large inductance will result in a small ripple current (e.g., curve 26) and a small inductance will result in a large ripple current (e.g. curve 28). Apparently, a large inductance is better for the inductor 16 of the converter 10 shown in FIG. 1, since large ripple output is not preferred. However, large inductance inherently induces a slow response speed, which makes the load current IL fail to follow the load variations well and, as a result, the transient performance becomes poor. In some applications, e.g., voltage regulator for a CPU, the load may change suddenly or violently. In particular, a CPU needs only a tiny current when it is idle and drains a huge current when it becomes very busy. If a smaller output inductor is employed in the converter to obtain a fast response speed to the load current transient (LCT), the steady state performance will become very poor because of the large ripple current. Therefore, due to the output inductor, it is difficult to design a voltage converter in consideration of its steady state and transient state performance simultaneously, and trade-off is necessary no doubt.
Two-Step DC-DC Power Conversion
On the other hand, two-step DC-DC power conversion had been proposed to provide flexible design for the voltage converter since the ratio of Vout/Vin in a two-step conversion becomes (Vout/Vm)×(Vm/Vin) by splitting the conversion into two stages, as shown in FIG. 3, where Vm is the modulated voltage between the two stages. In the converter system 30, for typical applications, the first stage 32 employs only a single-channel scheme for producing a suitable modulated voltage Vm serving as the input voltage of the second stage 34, and the second stage 34 employs a multi-phase scheme to obtain excellent output performance. For ease of explanation, the inductors L1 and L2 are specially drawn out of the rest of the conversion stages hereinafter. Similar to an ordinary DC-DC voltage converter, e.g., the one shown in FIG. 1, the first stage 32 includes a DC-DC conversion 36 and an output inductor 38, and the second stage 34 includes a DC-DC conversion 40 and an output inductor 42. Each one of the DC-DC conversions in a two-step system, e.g., the block 36 or 40, includes one pair of switching MOS transistors for each channel and their control and driver circuit. Obviously, based on the two-step voltage converter 30, a DC-DC voltage converter configured with more stages could be achieved by splitting the power conversion into more stages between the input and output voltages Vin and Vout, and the principles thereof remain the same.
The two-step DC-DC power conversion is also capable of minimizing the output ripple and reducing the LC value by splitting the power conversion into two stages, and optimizing the system performance of the second stage for that the second stage now needs only low-voltage devices and they have better response speed than high-voltage devices. An example of two-step DC-DC voltage converter is Taiwanese Pat. Application No. 091111366 issued to Dai et al. or its equivalent application, U.S. patent application under Ser. No. 10/442,077.
In the two-step conversion scheme 30 as shown in FIG. 3, the first stage 32 is so designed just for pre-modulating the voltage Vm, and the overall performance primarily depends on the second stage 34. In concerned with the system performance, a fundamental trade-off is made for setting each stage inductor value L as described in the following:dIL/dt=(1/L)(V1−V2),  1.                where V1−V2 is the voltage difference across the inductor L;V1−V2=Vin−Vm for the first stage conversion 32;  2.V1−V2=Vm−Vout for the second stage conversion 34;  3.ripple current Iripple=(1/L)(V1−V2)(Ton); and  4.peak current Ipeak=IL(DC)+(1/2)Iripple=IL(DC)+(1/2)(1/L)(V1−V2)(Ton),  (5).        where IL(DC) is the DC component of the inductor current IL and Ton is the “ON-time”, or Ton=Don/Fs, with Don denoting the ON-duty ratio and Fs denoting the switching frequency.        
It is further obtained two opposite conditions:                1. larger L and smaller (V1−V2) will result in smaller ripple current and peak current in steady state, but slower dIL/dt response; and        2. smaller L and larger (V1−V2) result in faster dIL/dt response, but larger ripple current and peak current in steady state.However, what is desired is “fast dIL/dt response when loading current is changing and low peak current in steady state operations”. Recently, the load transient (dIL/dt) is getting more and more critical for various applications, such as in CPU power supplies.PWM Control Loop        
As is well known, the high-side and low-side switches 12 and 14 in FIG. 1 are alternatively switched by the clock 20 that is generated by pulse width modulation (PWM) method, which is shown in FIG. 4. In such a PWM generator 44, an error amplifier 46 compares a signal VFB representative of the converter output voltage Vout with a reference voltage Vref so as to generate an error amp output Vc. The error amp output Vc is further compared with a ramp signal Ramps by a PWM comparator 48 so as to generate the clock 20 whose ON-duty Don is under controlled and to regulate the converter output voltage Vout to a desired value. In other words, the converter output voltage Vout can be controlled by adjusting the ON-duty Don or the ON-time Ton, and which can be implemented by combining a signal representative of tuning of the ON-duty Don or the ON-time Ton with Vref, VFB, Vc, or Ramps.
However, even though there are several control methods to tune the ON-duty and OFF-duty of the high-side and low-side switches 12 and 14 for the DC-DC voltage converter 10, the response speed to the load variations is always degraded by the stage delay in a two-step conversion system 30, since it is split into a two-stage mechanism. More specifically, referring to FIG. 3, the first and second stages 32 and 34 in the two-step voltage converter 30 have their respective PWM controls, i.e., the one in the first stage 32 modulates the voltage Vm, and the other in the second stage 34 regulates the output Vout. When a load current transient is occurred, the PWM control loop in the second stage 34 responds thereto first in order to maintain the converter output voltage Vout as stable as possible, which subsequently results in a transient impact to the modulated voltage Vm and thereby induces the PWM control loop in the first stage 32 to respond thereto further. Due to such a back-forward control scheme, the response speed of the whole system always fails to follow the load current transient in time.
Therefore, it is desired a method and apparatus to improve the response speed of a DC-DC voltage converter that employs a conversion scheme of two or more stages.